The Martian bow shock over solar cycle 23‐24 as observed by the Mars Express mission

The Martian bow shock position is known to be correlated with solar extreme ultraviolet irradiance. Since this parameter is also correlated with the evolution of the solar cycle, it is expected that the Martian bow shock position should also vary over such a period. However, previous reports on this topic have often proved contradictory. Using 13 years of observations of the Martian bow shock by the Mars Express mission over the period 2004 to 2017, we report that the Martian bow shock position does vary over the solar cycle. Over this period, our analysis shows the bow shock position to increase on average by 7% between the solar minimum and maximum phases of solar cycle 23-24, which could be even larger for more extreme previous solar cycles. We show that both annual and solar cycle variations play major roles in the location of the bow shock at Mars. Plain Language Summary The solar wind, which is the material ejected by the Sun, interacts directly with the upper atmosphere of Mars and its plasma environment. This interaction region is enclosed by a bow shock, a boundary where the solar wind is rapidly slowed so it can be diverted around the Martian space environment. The Sun's solar activity, the behavior of its material, and light output over time, varies over many different periods, resulting in a very dynamic Martian space environment. Studies of how the solar cycle, an similar to 11-year variation in the solar activity, impacts the Martian space environment have been inhibited by a lack of continuous observations of the environment. However, the European Space Agency's Mars Express mission has surveyed the Martian environment for >11 years. We use the longevity of this mission to describe how the Martian bow shock varies over this long period for the first time. In contrary to previous time-limited studies, we show that the Martian bow shock position is impacted by the solar activity, moving away from Mars as the solar activity increases. These results are important to future studies and missions that probe the interaction of the Martian atmosphere with the solar wind.